1. Field of Invention
The present invention generally relates to analysis of solutions. More particularly, the present invention relates to the analysis and detection of metals in acidic solutions.
2. Discussion of the Related Art
The matrix of a solution sample has a pronounced effect on the detection or quantification of trace analytes by modem analytical instruments. For example, an acidic matrix may obscure the detection and quantification of the metals. The difficulty of detecting, identifying, and/or measuring metals in acidic matrix exists for a number of different analytical tools. For example, an acidic matrix can obscure the detection and quantification of trace metals in ion chromatography. Acidic matrices are also problematic in mass spectrometry, such as inductively-coupled-plasma mass spectrometry.
Mass spectrometry is generally the technique of choice for measurement of parts-per-billion (ppb) and sub-ppb levels such as parts-per-trillion (ppt) of elements and compounds in solutions. For example, the present assignee, Metara, Inc., has developed an automated in-process mass spectrometry (IPMS) tool that for the first time allows users such as semiconductor manufacturers to detect and quantify the chemistry of wet process baths and cleaning solutions. Unlike traditional mass spectrometry instruments, the IPMS technique is automated and requires no human intervention. In contrast, the use of traditional mass spectrometers such as an inductively-coupled-plasma mass spectrometer (ICP-MS) requires hands on attention from highly-trained personnel.
The use of ICP-MS is typically “open loop” in that a calibration curve is first established by the users. In general, progressively concentrated (or diluted) solutions of the analyte of interest are processed through the ICP-MS instrument and the results recorded. For example, a 10 ppm solution may be processed, then a 20 ppm solution, and so on. Having established this calibration curve, a user may then analyze the solution of interest. By comparing response from the analyte to the calibration curve, a user may determine the amount of the analyte. If, for example, the response lies halfway between the 10 ppm and 20 ppm calibration curve recordings, a quantification of 15 ppm may be assumed.
But ICP-MS tools are prone to response shifts over time. Moreover, there may be response shifts caused by the difference between the matrices of the calibration standard and the sample. For example, if the acidic matrix shifts in composition, the calibration process must be repeated. These response shifts may be rapid, requiring frequent re-calibrations by experienced technicians. Thus, traditional mass spectrometry analysis was inappropriate for applications requiring continuous and unattended operation such as in semiconductor manufacture. In contrast to traditional techniques, however, IPMS instruments are “closed loop” and thus do not suffer from response shifts.
In an IPMS instrument, a processor controls an automatic sampling of the solution of interest, spiking the sample with a calibration standard, ionizing the spiked sample, processing the ionized spiked sample through the mass spectrometer to produce a ratio response, and analyzing the ratio response to determine the amount of an analyte in the sample. Unlike prior art open loop techniques, response drifts are not a problem—the drift affects the spike and sample in the same fashion and is thus cancelled in the ratio response. Thus, automated operation may be implemented without the necessity of manual intervention or recalibration. In addition, stable and reliable operation is assured by, in one embodiment, the use of atmospheric pressure ionization (API) such as electrospray to ionize the spiked sample. Moreover, the use of API enhances the characterization of molecular species. Furthermore, the IPMS technique is applicable to the analysis of analytes in either trace or bulk concentrations.
Despite the novel and advantageous properties of the IPMS technique, challenges remain in the detection and analysis of metals in an acidic matrix using this technique. Moreover, these challenges are also present in other analytical techniques such as ion chromatography. An example of an acidic matrix is a commonly-used cleaning solution during semiconductor manufacture that is known as Standard Clean 2 solution (SC2), which is a solution of hydrochloric acid (HCl), hydrogen peroxide (H2O2), and water in varying ratio. SC2 may be used to remove the metallic residues from the surface of silicon wafers by forming soluble chloride complexes. The most common ratio for SC2 used in semiconductor manufacturing is one part of 37% HCl to one part of 30% H2O2 to six parts of ultra pure water (UPW).
The continuous decrease in the geometry of semiconductor devices requires increased control of the contaminants in process solutions such as SC2. Control over the contaminants is important because SC2 comes in direct contact with the electronic circuitry during device fabrication. Thus, the quantitative determination and management of metallic contaminants in fresh and spent SC2 solutions is of immense importance, for example, in the optimization of semiconductor manufacturing yields.
Due to the high matrix of protons and chloride ions in the highly acidic SC2 solution, an online determination of trace levels of many metals is very difficult. Such a matrix obscures the analysis of metals in analytical instruments such as mass spectrometers or ion chromatographs. For example, because the metals will not be ionized in an electrospray ionization process, a corresponding mass spectrometer cannot measure or detect them. Moreover, even if other types of ionization such as inductively-coupled plasma ionization are used, the corresponding mass spectrometer cannot be subjected to such a harshly acidic matrix without instrument damage and/or interference problems. Thus, the analysis of metals in such matrices often involves the dilution of the matrix to reduce the matrix effect. But dilution of ultra trace concentrations of metals tends to dilute the metal concentrations to immeasurable levels. The background noise overwhelms the diluted ultra trace concentrations such that the mass spectrometer cannot detect or accurately characterize them. As an alternative, the matrix may be eliminated by heat and/or evaporation in an offline process. But volatile species such as boron or mercury are then lost. Moreover, it usually requires 24 to 48 hours to complete the sample preparation for the analysis in such instances. Accordingly, in most cases, if a problem is detected, such as impurities in the SC2, processing of defective product will have occurred for some time such that potential losses will be high.
Regardless of the analytical tool used for the analysis of acidic matrices, another problem with offline analysis is maintaining the integrity of the SC2 sample starting from collection to the end of analysis. For example, SC2 cleaning is typically done at elevated temperatures, between about 60° C. to about 75° C., and at this temperature the matrix of SC2 is dynamic in nature such that the components of the SC2 are continually reacting with other components and can change over time and with temperature. Thus, by the time the sample reaches a laboratory for analysis, the sample may not be in a representative formulation as it was at the time of collection. In addition, the SC2 matrix is a strong absorption media for airborne soluble contaminants such that if samples are exposed to air at any stage during sampling, transportation, or analysis, the matrix of the sample may be altered or contaminated. Moreover, the cleanliness of the sampling containers is important and a large amount of time and money is spent on cleaning sampling containers. Also, metals present in the sample solution may plate out or adsorb on the walls of the container. Thus, the amount of time the sample is allowed to sit in the sampling container before being analyzed can also affect the analysis outcome. It has been reported that even the cleanest of sampling containers can leach out many undesirable contaminants. Finally, offline elimination, neutralization, or modification of matrixes generally poses a high risk of contamination or sample modification that can affect the integrity of the sample and the accuracy of the subsequent analysis for all of the reasons stated above. Thus, offline analyses of metals in acidic matrices are problematic.
Accordingly, there is a need in the art for improved techniques for detecting and characterizing metals in acidic matrices.